Deepwater (also called subsea) processing is becoming more relevant in the field of oil and gas recovery, since deposits located below the ocean floor may often be made accessible by those techniques only. Therefore, it is necessary to adapt equipment for long step-outs (e.g., long distances), marginal and dispersed oil or gas fields, and for the high-pressure conditions of deep water. In the last years, large-scale seabed facilities for use in deep water were developed. Deepwater facilities may be designed to be operated for long step-outs with total reliability withstanding extreme pressures and temperatures. “Deepwater” is to be understood to describe situations of 300 meters and deeper, such as, in some embodiments, about 2,000 meters and deeper, or about 3,000 meters.
Deepwater processing facilities may include several electrically driven pumps and/or gas compressors to transport oil and gas over very long distances. Such pumps and compressors are driven by variable-speed or variable-frequency drives. The variable-speed drive may be connected to or part of a subsea power grid system via which the drive receives electric power for operation, or the drive may be directly supplied with electric power from an onshore power plant or an offshore platform, e.g. via an umbilical or sea cable. The variable-speed drive may be encapsulated in a pressure-resistant outer housing realizing an atmospheric environment, (e.g., an internal pressure of about 1 atmosphere), for the components of the variable-speed drive. The variable speed drives may have a complex design may not be easily scalable, and due to the atmospheric operation, may require an enclosure of considerable size and weight since the enclosure walls need to withstand pressure differences of up to 300 bar. This results in high production, transportation, and installation costs.
Conventional on-land variable-speed or variable-frequency drives may include a number of power cells arranged in a power cell system. Such power cells may be adapted for the application in medium-voltage or high-voltage variable-speed or variable-frequency drives. Medium-voltage may refer to a rated voltage greater than 690 volts (V) and less than 69 kilovolts (kV). Sometimes medium voltage may be a voltage between about 1000 V and about 69 kV. In many such systems, modular power cells are used. High voltage ratings exceed such medium voltage ratings, e.g., voltage ratings greater than about 69 kV.
The power cells used in conventional on-land variable-speed or variable-frequency drives may include one or more three-phase diode-bridged rectifiers, one or more direct current (DC) capacitors, and one or more H-bridge inverters as disclosed, for example in U.S. Patent Publication No. 2007/0048561 A1 for variable-frequency drives. The rectifier converts the input alternating current (AC) voltage to an essentially constant DC voltage that is supported by the capacitors that are connected across the rectifier output. The inverter transforms the DC voltage across the DC capacitors to an output using pulse-width modulation of the semiconductor devices in the H-bridge inverter.